1. Field of the Invention
The invention relates generally to synchronization techniques for wireless communications, and more particularly, to resynchronization when recovering from a sleep mode for wireless communications.
2. Description of the Related Art
When a mobile device is powered on and trying to connect to a service network, it first synchronizes with the service network by aligning its system timing with the timing pattern of the service network before establishing wireless communication connection with the service network. Take the Global System for Mobile Communications (GSM) for example. When powered on, a mobile station (MS) detects a cell in the GSM service network with the best received signal strength in the area and then searches for the synchronization channel (SCH) of the cell. The MS may obtain the timing pattern of the cell by identifying the frame boundary and slot boundary on the SCH of the cell. Subsequently, the MS may align its system timing with the timing pattern of the cell to synchronize with the cell, and accordingly obtain wireless communication services from the GSM service network if necessary. Generally, a wireless modem in the MS may maintain a modem counter with the obtained timing pattern of the cell. After successfully camping on the cell, the MS may be configured to enter a sleep mode if there's no potential operation for wireless communications in a forthcoming period of time. The MS only needs to wake up from the sleep mode every known paging cycle to check if there's a dedicated paging request or an update of the system information broadcasts. For an MS operating in the sleep mode, the wireless modem therein is shut down, and the processing unit therein, such as a central processing unit (CPU) or a micro control unit (MCU), switches from a high-rate clock to a low-rate clock to save on the consumed power. Since the wireless modem is shut down in the sleep mode, the modem counter is stopped and does not keep track of the timing pattern of the cell. Thus, resynchronization with the cell is necessary for the MS to align its system timing with the timing pattern of the cell when recovering from the sleep mode.
FIG. 1 is a timing diagram illustrating the resynchronization for an MS recovering from the sleep mode. As shown in FIG. 1, the system timing of the MS is aligned with the timing pattern of the base station (BS). At time t1, the MS determines that no potential operation for wireless communications is expected in a forthcoming period of time, and issues a trigger signal to indicate that the wireless modem can enter the sleep mode for N seconds. After the trigger signal is issued, a short period of time (i.e. the time interval between t1 and t2) is occupied by the software and hardware modules of the MS to prepare to switch to the sleep mode from the non-sleep mode. At time t2, the MS officially enters the sleep mode in which only a low-rate clock is running to sustain the basic functions of the MS, and the timing pattern of the BS is not count in the sleep mode. It is noted that, although the predetermined N seconds has not yet passed, the MS needs to wake up in advance from the sleep mode at time t3, to perform calculation of the current time in the timing pattern of the BS, so that the MS may smoothly resynchronize with the BS at exactly N seconds after time t2. Specifically, the resynchronization calculation includes division operations which consume time and power and require the CPU to run at a full speed. As a result, the early waking-up of the MS consumes extra system power for the resynchronization calculation for timing resume.